Enable reclaim throttling by default, with "Slow" setting

Per findings in experiment as shown in links below, this setting
improves jank and memory pressure, without any impact to memory
consumption.

This is with last ramp-up: http://cl/484073439 (google).
Feature helps users who are in most need (high jank, high PSI).

The "Slow" group had the best improvement of all three groups in all metrics use (jank, memory pressure, memory used), so picked metrics from that group.  Below are stats from "Slow":

- Overall
https://uma.googleplex.com/p/chrome/variations?sid=8c62d05904295477aa61b3b5e652dca4
* -0.05% jank
* -3.57% mem pressure at 95th percentile

- Low-memory devices, RAM <= 4GB
https://uma.googleplex.com/p/chrome/variations?sid=0254b7d87dae70b8b52ad39ff35b26fd
* -0.05% jank
* -3.90% mem pressure at 95th percentile

- Medium-memory devices, 4GB < RAM <= 8GB
https://uma.googleplex.com/p/chrome/variations?sid=d7fb7c6a32ecded1b6e49b60f9eeed2a
* -0.10% jank
* -0.77% memory footprint at 95th percentile

- High-memory devices, RAM > 8GB
https://uma.googleplex.com/p/chrome/variations?sid=2c9e5772fe5b5d1e84cfa80b6dc0075d
* +0.23% in mem pressure at 95th percentile
* No other significant changes

BUG=b:227251697
TEST=manual + gtest_filter=WorkingSetTrimmerPolicyChromeOSTest.*:WorkingSetTrimmerPolicyArcVmTest.*'

Change-Id: Ie84506e02d1c8b188ad4e90bae3fffc81b9fad36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4054748
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Auto-Submit: Alexandre Marciano Gimenez <raging@google.com>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1078392}